The present invention generally relates to field current discharge and, more specifically, to systems and methods for discharging current to eliminate damage caused by generator overvoltage when a load is removed.
The output of a generator or alternator may be regulated by comparing the voltage at a point of regulation (POR) with a reference voltage and using a voltage regulator to maintain the output voltage at a desired level. Some systems now require generators that limit overvoltage to about 150V rms for 115 V AC electrical systems and to about 300 V rms for 230 V AC electrical systems.
A conventional approach to overvoltage protection is to monitor voltage levels and disconnect the generator or alternator from the power supply when an overvoltage is detected. This approach, however, is too slow to provide effective control and protection against overvoltage conditions.
For synchronous alternator applications, fast field current discharge is required to eliminate the damages caused by overvoltage that occurs during load removal. Voltage regulator circuitry and the field winding exist to regulate the terminal voltage of the generator to meet the predetermined specifications. During the load removal, the generator terminal voltage increases due to reduced losses. Therefore, the voltage regulator is engaged to reduce the field current. The field power supply is unidirectional, so the stored energy in the form of the field current has to be dissipated in the field resistance. Since the field resistance is very small, the recovery time is large and it takes a long time to reduce the field current and, consequently, a longer overvoltage appears on the generator terminals.
One conventional approach to overvoltage protection uses a discharge resistance to dissipate the field current energy. The larger the discharge resistance, the shorter the recovery time. Discharging the field current through a high resistance value results in high voltage across the discharge resistance, which could exceed the aircraft and components safe working voltage. In addition, a special resistor, with high pulse energy rating, is required, which adds to the circuit cost.
Although discharging the field current through a resistance can be fast, the discharge time is limited by the voltage across the given resistance. For ideally fast discharge time, a prohibitively large resistance may be necessary. Therefore, due to the inability to use such large resistances, the discharge time would not be as fast as required. Additionally, a large electromagnetic interference pulse is produced during a resistive discharge which could disturb surrounding electronic equipment.
Referring to FIG. 1A, there is shown a generator field circuit 10 having a constant direct current (DC) voltage source, +VDC and −VDC. The voltage regulator (VR) 12 provides a command signal 14 to the pulse width modulator (PWM) circuit 16 which triggers a PWM-transistor 18. The transistor 18 is used to chop the VDC, controlling the field voltage and consequently the field current 30. During load removal, the generator output terminal voltage 20 rises and the voltage regulator 12 commands the PWM circuit 16 to reduce the field current 30. The field current 30, however, keeps circulating through the free-wheeling diode 22.
Referring to FIG. 1B, there is shown a conventional circuit 24 for discharging the field current 30 quickly using a resistance 26. A field discharge circuit 28 is added in series with the free wheeling diode 22 in the field circuit 10 (see FIG. 1A) to quickly reduce the field current 30. The field discharge circuit 28 may include the resistor 26 and a field discharge transistor 32. As discussed above, the discharge time is limited by the voltage across the resistance. Additionally, a large electromagnetic interference pulse is produced during a resistive discharge which could disturb surrounding electronic equipment.
As can be seen, there is a need for a field current discharge circuit and method that may quickly reduce the field current without requiring a large resistance which may emit electromagnetic interference pulses.